Inside HIRF Testing: Ensuring Aircraft Safety in High-Power RF Environments
Electro Magnetic Applications, Inc. (EMA) is investing in the future of electromagnetic environmental effects (E3) engineering by training the next generation of specialists.
“We’re really solving a major gap in the industry,” EMA Chief Technology Officer Justin McKennon said. “There needs to be a mechanism to transfer knowledge so that we can do more testing and help more people.”
To address that need, more than a dozen EMA scientists and engineers gathered at the company’s laboratory in Pittsfield, Massachusetts, for an intensive, hands-on training focusing on high-intensity radiated fields (HIRF) testing for aerospace certification. Training covered equipment setup and calibration for accurate measurements, common pitfalls in RF measurement, and safe operation of the equipment.
“There’s a lot of different realms of physics that are involved,” McKennon explained. “Hearing it, talking about it, and reading it are really no substitute for getting hands-on.”
However, before any aircraft testing can begin there is a fundamental question to answer: What exactly is HIRF testing?
Understanding HIRF in Aircraft Certification
HIRF describes powerful radio frequency (RF) energy generated by external sources such as radar systems, radio transmitters, and cellular towers. When aircraft fly through these environments, the energy can interfere with onboard electrical and electronic systems, potentially causing disruptions, damage, or unexpected behavior. To prevent this, engineers perform HIRF testing to verify that flight-critical systems remain safe and fully functional, even when exposed to high-power RF environments. Industry standards such as SAE ARP5583A define how to conduct this testing, which treats the aircraft as an integrated system and evaluates its overall susceptibility to electromagnetic energy.
Key Factors Influencing Aircraft Susceptibility to HIRF
An aircraft’s susceptibility to HIRF is influenced by both the intensity of the RF energy it encounters and the ability of its onboard systems to withstand electromagnetic exposure. Table 1 summarizes the external RF power incident on the aircraft, while Table 2 describes onboard responses to RF exposure.
Table 1. RF power incident on aircraft.
Table 2. System sensitivity to RF fields.
Table 3 provides examples of common HIRF environments, illustrating how RF exposure can vary across different operational scenarios.
Table 3. Examples of typical HIRF environments an aircraft may encounter.
Conducting a HIRF Safety Assessment
A HIRF safety assessment identifies all potential adverse effects and should be integrated into the overall aircraft safety assessment.
The five key questions for a HIRF safety assessment are:
- What are the functions being performed?
- What constitutes the system performing the function?
- What are the consequences of failure for the function and system?
- What are the adverse effects that would prevent continued safe flight and landing?
- What is normal operation, a timely manner, and automatic recovery?
Safety assessment steps are:
- Identify aircraft functions
- Determine the consequences of failures associated with these functions
- For catastrophic failures, identify the electrical and electronic system that performs each function
- For hazardous and major failures, identify the equipment that performs each function
- Determine adverse effects for the systems that are associated with these failures
Because HIRF represents a common-mode environment, system redundancy alone does not typically mitigate HIRF-related failures. Failure condition terminology used in this assessment is summarized in Table 4.
Table 4. AC 20-158 failure conditions terminology
The EMA training session focused on Low Level Swept Current (LLSC) and Low Level Swept Field (LLSF) measurements.
LLSC Test Methods
LLSC testing measures the electrical currents induced on aircraft wiring harnessing by external electromagnetic fields across a wide frequency range. This test helps engineers identify how RF energy couples into the system and reveals potential vulnerability paths.
Equipment calibration starts with a D-dot sensor at the center of the test site, where the center of the aircraft will be during actual testing, Figure 1. Ground bounce is taken into consideration by moving the sensor up and down during the calibration process.
Figure 1. Calibration of D-dot sensor of LLSC testing.
During testing, engineers measure current at multiple locations, orientations, and polarizations. The aircraft is moved to the center of the test site, sitting on isolating pads, Figure 2. Short cables (<1m) are routed between the harness and fiber optic unit. The aircraft doors and access panels are closed. The same transit systems and settings must be used for calibration and testing. Testing takes two to three hours to calibrate and eight hours to sweep everything.
Figure 2. LLSC testing setup.
Engineers combine worst-case results and normalize them to an incident field strength of 1 V/m. The final output is a transfer function curve, expressed in mA/V/m, which shows how strongly external fields couple to the aircraft’s wiring.
LLSC testing helps set appropriate conducted susceptibility test levels. Engineers do this by scaling the measured transfer function to match the expected external HIRF environment.
LLSF Test Approach
LLSF testing focuses on how RF fields couple onto aircraft structure and wiring. This method effectively measures the aircraft’s RF attenuation, making it a practical way to evaluate overall shielding effectiveness.
As with LLSC, transmitters and receivers must be calibrated. Transmit antennas are bilog and horn, Figure 3. Receiver antennas are usually small active dipole up to ~2 GHz and a mini bicon from ~1Ghz to 18GHz.
Figure 3. Equipment used in LLSF testing.
During testing, engineers collect data across multiple test points and polarization to retain the worst-case results. For this test it is best to choose angles around the aircraft that would create the highest coupling, such as pointing the antenna at seams and windows. A mechanical mode stirrer is set up near where equipment will be installed to redistribute electromagnetic fields. It needs to have different sized paddle wheels as well as be a battery powered unit with a housed motor to minimize emissions, Figure 4.
Figure 4. Mechanical mode stirrer inside the aircraft during LLSF testing.
Following the method outlined in ARP5583A Section 6.4.5, it is acceptable to average the measured data when determining final shielding effectiveness. The resulting values provide insight into how well the aircraft structure protects internal systems from external RF energy.
LLSF testing supports the definition of radiated susceptibility test levels. In this case, engineers multiply the measured average HIRF attenuation by the applicable external HIRF environment to determine realistic test conditions.
Both LLSC and LLSF tests can be used to:
- Identify circuits that are vulnerable to HIRF
- Validate computational and analytical models
- Define needed design improvements, such as adding shielding or filtering, rerouting cables, or improving bonding and grounding
- Demonstrate compliance, including FAA requirement to prove that critical systems continue to function in a HIRF environment
- Support overall system risk assessments
High-Fidelity HIRF Simulation
Software simulation can be used to validate the results of physical testing. Figure 5 illustrates the Ansys EMC Plus model and corresponding horizontal and vertical shielding effectiveness results from the front-center view of the training setup.
Figure 5. Ansys EMC Plus simulation model and LLSF shielding effectiveness results of training setup.
EMC Plus uniquely models complex cable interactions, accurately distributing HIRF energy across harnesses and conductors while accounting for ohmic losses and skin depth effects. The resulting simulations of real aircraft cable bundles demonstrate dramatic improvements in fidelity. Dig deeper into the benefits of advanced simulation here.
HIRF Testing Without Compromise
“Very few companies, if any in the world, have the ability to perform HIRF testing on an aircraft,” McKennon said.
He explains that executing this type of testing requires more than specialized equipment, it demands highly mobile test systems and a team with deep expertise in both operating that equipment and interpreting results.
“That’s what we have,” he said. “That’s unique.”
EMA delivers end-to-end HIRF testing services, including:
- Comprehensive test planning, execution, and reporting
- Certification support, including Designated Engineering Representative (DER) services
- Early-stage HIRF simulation and design support
- Troubleshooting to provide solutions to problems
- On-site support for design team
- Configuration management to ensure simulation and test traceability
“Being a small business and being agile really gives us a leg up,” McKennon said. “Because we’re easy to work with, we can get there quickly and we bring with us the whole wealth of EMA expertise that goes along with that.”
To learn more about EMA’s HIRF testing capabilities or to talk about your program needs, contact EMA today.
